1. Field of the Invention
The present invention relates to a scanning probe microscope and scanning method capable of obtaining information about the surface characteristics such as the surface topography of a sample by scanning a probe tip over a sample surface while vibrating the tip.
2. Description of the Related Art
As is well known in the art, scanning probe microscopes such as atomic force microscopes and scanning tunneling microscopes are known as instruments for measuring a sample such as an electronic material in a microscopic region and imaging the surface topography of the sample or measuring information about surface characteristics. Various kinds of instruments are offered as this type of scanning probe microscope. One known instrument is an ultra-low force atomic force microscope (AFM) for detecting the surface topography of a sample by scanning a probe tip parallel to a surface of the sample while vibrating the tip vertical to the surface (for example, see Patent Reference 1).
This ultra-low force AFM is equipped with a probe mounted at the front end of a lever arm. This probe can be scanned parallel to a sample surface. Furthermore, the tip of the probe can be vibrated at or near its resonant frequency.
In this ultra-low force AFM, when the surface topography of the sample is measured, scanning is performed while repeatedly tapping the sample surface by the probe vibrated as described above. Consequently, the vibration amplitude of the tip of the probe varies according to the topography (i.e., roughness) of the tapped sample surface. The ultra-low force AFM measures the surface topography of the sample by measuring the variations in the vibration amplitude.
The vibration amplitude of the probe tip is set to a sufficiently large value (e.g., 100 to 200 nm) to prevent the tip from adhering to the sample surface, for the following reason.
Under atmospheric conditions, a thin liquid layer, i.e., an adhesive water layer, exists on the surface of a substance due to moisture in the air. That is, as shown in FIG. 10, an adhesive water layer is also present on the sample surface. Therefore, when the probe tip is brought into proximity or contact with the sample, the amplitude gradually decreases while drawing a smooth curve as indicated by C1 in FIG. 11 by the effects of the force exerted between the tip and sample such as Van der Waals force. On the other hand, when the probe tip is moved away from the sample, the tip is captured by the surface tension of the adhesive water layer and pulled. Consequently, the tip draws an irregular curve C2 different from the curve C1. For this reason, it has been difficult to separate the probe tip from the adhesive water layer with small amplitudes. The probe tip is vibrated at large amplitudes as mentioned previously to solve this problem.
Furthermore, it is known that when a sample of deep topography is measured by the ultra-low force AFM, it is necessary to set the vibration amplitude large (more than 100 nm) as mentioned previously such that the tip can easily trace the topography (for example, see Non-Patent Reference 1).
In addition, where the topography of the sample has a steep incline, if the force with which the incline is tapped increases, slip produced on the incline increases (for example, see Non-Patent Reference 2). Also, lateral force undergoing from the incline increases bending of the probe and cantilever (lever arm).
In another known scanning probe microscope, when the probe tip is scanned parallel to a sample surface, the tip is kept at a distance sufficiently greater than the distance necessary for measurement of a physical property of the sample from the sample surface (for example, see Patent Reference 2).
This scanning probe microscope is equipped with a Z piezoelectric element for moving the probe vertical to the sample surface, the probe being capable of being scanned parallel to the sample surface. The Z piezoelectric element is elongated or shrunk by applying a voltage to it. Thus, the distance (or height) between the probe and sample surface can be adjusted. Usually, the Z piezoelectric element is shrunk when the tip is in a position sufficiently remote from the sample surface as mentioned previously. This position is set as the initial state.
In this scanning probe microscope, in a case where the surface topography of the sample is measured, the probe tip is scanned. When the tip reaches a measuring point, the scan is stopped and, at the same time, a voltage is applied to the Z piezoelectric element. Consequently, the Z piezoelectric element elongates to move the probe tip toward the sample surface. When the tip comes in close proximity to the sample surface and enters a tunnel area, a tunneling current flows between the probe tip and the sample. The microscope measures the current. After the measurement, the voltage applied to the Z piezoelectric element is set to 0 V. The Z piezoelectric element then shrinks back to its initial state. That is, the tip is in a position sufficiently more distant from the sample surface than a position giving the distance necessary for the measurement. After returning to this initial position, the tip is scanned again. Then, the microscope repeats the process described so far. The scanning probe microscope measures the surface topography of the sample by measuring the tunneling current at each measuring position.
Patent Reference 1: Japanese Patent Number 2732771 (paragraph numbers 0015-0037; FIGS. 1-11)
Patent Reference 2: Japanese Patent Number 2936545 (page 2, left column, from 16th line from below to page 2, right column, 5th line from above)
Non-Patent Reference 1: B. Anczykowski, et al., How to measure energy dissipation in dynamic mode atomic force microscopy, Appl. Surf. Sci., 140 (1990), 376 (page 379, lines 16-Z26) Non-Patent Reference 2: T. Morimoto et al., Atomic Force Microscopy for High Aspect Ratio Structure Metrology, Jpn. J. Appl. Phys., Vol. 41 (2002) 4238 (page 4240, left column, lines 9-17 and FIG. 8)
In the ultra-low force AFM described in the above-cited Patent Reference 1, scanning is performed at large amplitude while tapping the sample surface repeatedly such that the probe is not captured in the adhesive water layer on the sample surface. At this time, however, as described also in Non-Patent Reference 1, the dissipation energy produced by tapping the sample surface by means of the probe is in proportion to the squares of the vibration amplitude. Therefore, if the vibration amplitude is increased, there is the danger that a collision occurs between the probe and sample, producing a damage.
Furthermore, as described also in Non-Patent Reference 2, if the force with which the incline is tapped is increased, slip occurring on the incline increases. In addition, bending of the probe and cantilever due to lateral force undergoing from the incline increases. Therefore, it has been difficult to accurately measure the topography of a steep incline.
To eliminate this problem, if the vibration amplitude of the probe is reduced,the probe adheres to the sample surface. That is, the probe is captured in the adhesive water layer. This produces the problem that measurements cannot be made precisely.
Furthermore, if the scan rate is increased, the probe cannot follow the topography of the sample surface and thus it is difficult to make measurements precisely. Consequently, limitations are imposed on the scan rate. This deteriorates the throughput. In addition, it takes a long time to make measurements. Especially, where the scanned area is increased, the measuring time is increased.
Additionally, where the servo gain that is used as a parameter in controlling the distance between the probe and sample is increased, hunting oscillation occurs, making it difficult to perform precise measurements.
In the scanning probe microscope described in the above-cited Patent Reference 2, when the probe tip is scanned, the voltage applied to the Z piezoelectric element is set to 0 V. Scanning is performed after the tip has been returned to its initial position, i.e., the tip is placed sufficiently remotely from the sample surface. Therefore, the tip must move long distances in reciprocating between the tip and the sample surface. Hence, it takes long times to make movements. Therefore, it takes a long time to perform measurements. This produces the disadvantage that the throughput is deteriorated. Especially, since the probe tip is so set that it always returns to the same position (initial position) regardless of the topography of the sample surface, it may take a longer time according to the topography of the sample surface. In consequence, it also takes a longer time to perform measurements, and the throughput is deteriorated.